The hydrometallurgical recovery of metals often comprises the following stages: the leaching of a concentrate or ore, liquid-liquid extraction and the precipitation or reduction of the metal. Leaching may be bioleaching or dilute acid leaching, from which the aqueous solution is routed to liquid-liquid extraction. In extraction, organic extraction solution is mixed, in an extraction cell (mixer-settler) or in a column, into an aqueous solution that contains a metal, usually in ion form or as a compound along with several impurities. The valuable metal to be refined reacts selectively with the organic extraction solution, whereby it is separated from the aqueous solution into the extraction chemical in a pure form. The aqueous solution depleted of valuable metals, i.e. the raffinate, is routed back to concentrate/ore leaching. The valuable metal or substance bound to the extractant can then be separated from the organic solution back into an aqueous solution (stripping) with the inverted chemical reaction to extraction, and can then be recovered again from there as a product, for instance by precipitation or reduction to metal. Reduction may be electrolysis for example.
The extraction process is thus the mixing together of liquids that are physically insoluble in each other into droplets or a dispersion in the mixing section of the extraction apparatus, and after chemical mass transfer, the droplets in the dispersion are made to coalesce back into the original layers of liquid in the settling section or settler. Intensive mixing or a significant change in the surface chemistry conditions of the process may result in very small droplets, which require a lot of time to disengage to their own liquid phase. These droplets do not necessarily have time to disengage in the actual settling section of the extraction step, but move further along the process with the other phase.
The entrainment of extraction solution in process stages occurring in the aqueous phase such as leaching or electrolysis (electrowinning) causes process disturbances. Bioleaching is particularly susceptible to disruption, because organic extraction solution is toxic to the bacteria that maintain bioleaching. In addition, the purity of metal produced by electrowinning suffers from the extraction solution that accumulates in the electrolysis tanks. Therefore, electrolyte that enters electrolysis must also be purified carefully from droplets of extractant. A maximum of only 3-5 ppm extraction solution is permitted in the electrolyte.
For example, in the hydrometallurgical recovery of copper, a mixer-settler apparatuses are used which are principally laid out horizontally. Their operation has improved in recent years to the extent that the amount of extraction solution entrained in the aqueous solution is in the range of about 10 ppm. However, it has been found that the permanent reduction of the amount of entrained droplets below 5 ppm cannot be accomplished using only a mixer-settler apparatus.
The separation of small droplets from another solution occurs using the droplet coalescence principle. When droplets are made larger, they can be separated from another solution due to the effect of gravity. There are several types of droplet coalescers, for instance plate coalescers, fibre/mesh coalescers, packed bed coalescers and membrane coalescers.
Nowadays, for instance, a kind of packed bed organic droplet coalescing device, which is in fact a pressure filter, such as the one described in U.S. Pat. No. 6,015,502 is used for purifying the electrolyte. A filler such as anthracite is used in the pressure filter to bind the droplets. The filler is regenerative at regular intervals, so that its pores do not get clogged too much with organic solution. In practice, several devices connected in parallel are required for purification, for example four to six units in a large copper extraction facility. The apparatus is expensive and its operating features are complicated. Since the flow direction of solution has to be changed from time to time, this results in the fact that solutions from different operating stages are mixed together disadvantageously. At the same time, some of the electrolyte and extraction solution is also lost.
The amount of raffinate, the stream of aqueous solution from the extraction stage that is routed back to leaching, is considerably greater than the amount of electrolyte in contact with organic solution in the stripping stage. Even if the apparatus described in U.S. Pat. No. 6,015,502 were used to purify electrolyte, its use in raffinate purification would not be economically viable in practice. Tanks to purify the raffinate are proposed, into which slotted plates are placed according to various embodiments. The distance between the plates is usually over 10 mm and when the size of the droplets of extraction solution is less than 50 microns, the cleaning effect generally remains modest, far less than half the amount of extraction solution contained in the raffinate.